Oligonucleotides have been shown to interact with mRNA and other components associated with transcription. By virtue of such interaction, natural and synthetic oligonucleotides have become valuable tools for research and important agents in diagnostic, therapeutic and other applications. By virtue of their many uses, there is a demand for improved oligonucleotides, e.g. oligonucleotide analogs.
Oligonucleotides and oligonucleotide analogs have been used in a number of areas of research. In genomic research oligonucleotides have been used as probes and primers. Oligonucleotides are also useful in diagnostics because of their ability to specifically hybridize to nucleic acid sequences of interest in the etiology of a given disease. Oligonucleotides are also being tested as therapeutic moieties in the treatment of disease states in animals and man. For example, oligonucleotide therapeutic compositions have been identified that are capable of modulating expression of genes implicated in viral, fungal and metabolic diseases. It has now become routine to synthesize oligodeoxyribonucleotides and oligoribonucleotides having hundreds of base pairs (bp) by solid phase methods using commercially available, fully automatic synthesizers. Thus, oligonucleotides are of increasing importance, and have great utility in biotechnology and medical applications.
Unmodified oligonucleotides, i.e. natural phosphodiester linked oligonucleotides, are impractical for some in vivo applications because they either have short in vivo half-lives or suffer from a limited ability to penetrate cell membranes. Several variations in the polynucleotide backbone have been proposed to overcome these limitations, including the use of methylphosphonates, monothiophosphates, dithiophosphates, phosphoramidates, phosphate esters, bridged phosphoramidates, bridged phosphorothioates, bridged methylenephosphonates, dephospho internucleotide analogs with siloxane bridges, carbonate bridges, carboxymethyl ester bridges, acetamide bridges, carbamate bridges, thioether, sulfoxy, sulfono bridges, various "plastic" DNAs, .alpha.-anomeric bridges, and borane derivatives. These analogues have diverse properties, and several of these derivatives have been shown to be superior to natural oligonucleotides for particular applications.
One important oligonucleotide analogue is a class of compounds known as peptide nucleic acids (PNAs).
See PCT application PCT EP92/01219, filed May 19, 1992 and published Nov. 26, 1992 as WO 92/20702, and PCT application EP/01220, filed May 19, 1992, and published as WO 92/20703. See also U.S. application Ser. No. 108,591 filed Aug. 27, 1993, which is assigned to an assignee of this Application. Each of the foregoing applications are hereby incorporated by reference in their entirety. PNAs have enhanced hybridization to complimentary DNA and RNA strands as compared to natural oligonucleotides and most other known oligonucleotide analogues, and also have enhanced nuclease and protease stability. PNAs have uncharged, amide linked backbones instead of the charged phosphodiester backbone of oligonucleotides. This neutral backbone has proven to be especially useful in diagnostics, wherein PNAs have been hybridized to complementary DNA strands, and the resulting duplexes have been analyzed by capillary electrophoresis.
Consequently, there remains a need in the art for stable compounds that can form double-stranded, helical structures mimicking double-stranded DNA.